The present invention relates to a multi-stage diffuser-type centrifugal pump, and, more particularly, to a multi-stage diffuser-type centrifugal pump suitable for supplying under pressure a low-temperature fluid such as, for example a liquified gas.
Conventionally, as disclosed in, for example "Submerged Motor Pump for Cryogenic Plants" HITACHI REVIEW, Vol. 32 (1983) No. 1 and Japanese Patent Laid Open No. 183892/1983 a pump of the aforementioned type is disposed in a suction pot with the arrangement being such that the pressure of a fluid flowing from a suction pipe into the suction pot is increased by impellers disposed in a plurality of stages driven by a motor section, and the fluid passes through a discharge communicating passage and is discharged from a discharge pipe.
The diffuser has a diffuser vane and a return vane and is accommodated in a stage with the stage including a flat plate section for holding the stage in the axial direction and an outer barrel section forming a pressure boundary with the outside of the pump.
In the above-described construction, each stage is clamped by a suction casing and a motor casing and is tightened by a plurality of tightening bolts with as to be sealed from the outside of the pump, so uniform and large tightening forces being required for the tightening bolts. Thus, considerable experience and skill is required in tightening operation since, should the tightening bolts be unevenly tightened, the stages are fixed in a bent state, so that gaps are provided at the respective joint surfaces of the outer barrel sections of the stages thereby resulting in an increase in the amount of liquid that leaks to outside the pump under pressure causing the pump efficiency to decline. If the amount of uneven tightening is considerably large, fixing members for the stages and diffusers, and the axes of rotary bodies including the shaft and the impellers, do not align with each other, thereby resulting in accidental contacting of the component during operation of the pump. For this reason, it has been necessary to carefully tighten the tightening bolts. Additionally, since the tightening bolts are arranged with intervals provided therebetween in the direction the outer periphery of the stage, another disadvantage resides in the fact that the outside diameters and thicknesses of the suction casing and the motor casing become large. Furthermore, a large suction casing for accommodating a pump has also been required. Accordingly, there has been a problem in that the pump tends to become large in size and heavy in weight.
Japanese Patent Laid-Open No. 167895/1983 another an apparatus wherein sectional type stages, sectional type diffusers, and impellers are accommodated in the state of stages in a cylindrical barrel, and the diffusers, stages, and the barrel are formed of materials having different coefficients of thermal expansion, e.g., austenitic stainless steel and an aluminum alloy to ensure that the outer peripheral surface of each stage and the inner peripheral surface of the barrel are brought into close contact with each other by making use of the difference in the coefficients of thermal expansion.
With such a prior art, the diffusers, the stages and the barrel must be fabricated with metals having different coefficients of thermal expansion, so that fabrication is troublesome, and, since either the diffusers and the stages or the barrel are fabricated with a metal having a greater coefficient of thermal coefficient, it is very difficult to obtain sufficient strength, making it impossible to fabricate a multi-stage diffuser-type centrifugal pump having high strength.